Engine Parts and Functions

 Marine Diesel Engine Parts and Functions

Previously we tackle the operating principle of a diesel engine. No matter in what field diesel engine is used, small or large engine and different engine builders or design - the operating principle will be the same. In some phases like in the construction, design, size, and parts there will be differences that due to surrounding circumstances and necessity as well, exist. Well even in parts naming, obviously a student or professional can observe. Technical terms are being used like fuel spray valve, fuel oil injector, injector, spray valve which refer to the same part and component of the said engine. Another one is the crankcase cover which is another way of naming it is called the crankcase window. These are just a few examples to keep one comprehension flexible of the by subject. 

Parts and Respective functions

Below is a figure showing some of the major parts of a four-stroke cycle diesel engine. We will have to identify each part and the respective functions:

Fig. 1
Parts of a Marine Diesel engine

This is Inline, a Vertical marine four-stroke diesel engine. These are just a few of the major parts in a diesel engine exposed that can be viewed outside. Well, let's start.

1. Turbo Charger 

This part is installed at the farthest end of the exhaust manifold. The exhaust gases
from cylinders pass-through this component. It has two sets of blades. One is turbine blading on the exhaust side and the other one is blower blading located in the inlet manifold. The two sets of blades are attached to the ends of one shaft respectively. Turbine side and blower side so to speak as they are described. 


As the exhaust gases pass through the turbine blades, the latter rotate, and since the blower assembly
is also secure at the other end of the rotating shaft, the blower blades also turn to suction ambient air channeled to intake ports and so, to the respective cylinder.
Therefore, the turbocharger the one that supplies additional air to the engine cylinder seeing to it there is an adequate amount of oxygen needed to ignite the fuel on the power stroke.

Advantages over Naturally Aspirated engine

  • It promotes a cooler engine thereby, suitable for long voyages.
  • It is proven that the turbo-charged engine has higher fuel oil efficiency which means, it affects lower fuel consumption than the non-turbo-charged engines.
  • The engine operates smoother and quieter.


  • Additional maintenance and space
  • Additional cost and expenditure

2. Piston

This part is hidden. It is installed and acts inside the cylinder. since this is inside the cylinder it conforms to the shape of the cylinder. It is connected to a piston rod. While the topmost part of it is exposed to hot combustion gases, is cooled by the lubricating oil. the piston moves up and down in the vertical engine. In the case of the horizontal engine, to and fro. in the case of V-type, diagonally. In radial engine installed in the aircraft, depending on the position of the cylinder.


The piston catches the pressure of combusting gases as fuel is ignited. This prompts the piston to move away from the combustion space thus reciprocating motion is created. The motion is communicated until the motion reaches the crankshaft and other parts. Do understand that the initial movement is done through the piston causing both the reciprocating and rotating masses to do so. Therefore work is created.

3. Cylinder

The cylinder, cylinder liner, or sometimes liner, accommodates the piston. It is where the piston performs its job to convert heat energy into useful work. The inner portion of the cylinder is smooth to provide as possible zero friction to the piston activity thus untimely wear-out of piston rings is avoided. Large engines do not rely on swash-type of lubrication. Instead, it has two opposite oil holes or other means to provide adequate lubrication and cooling to the piston.


Cylinder accommodates piston and provides a very smooth (frictionless as possible) inner surface for the piston to move by providing lubricant passages for lubrication and cooling. Cylinder gives a leakproof space to combustion for expansion where expanding heat energy will concentrate its force on the piston top to move along the line of force. 

4. Cross-head

Cross-head connects the piston rod and connecting rod. The cross-head can be found in large engines. This part provides enough angle so that the piston rod will not strike the inner sides of the cylinder therefore, it's in the lower portion of the cylinder. So that its location is between the piston rod and connecting rod. Therefore the Cross-head connects the piston rod and the connecting rod in large engines since medium to small engines in general, do not have Cross-head. 


As said it serves as a link between the piston rod and connecting rod in such a way that the piston will have a free movement side-to-side at an angle, without hitting the bottom portion of the cylinder. 

5. Connection or Connecting rod

This connects from the cross-head to the crankshaft. Like the piston and the piston rod, it is the primary passage in the transmittal of energy from the piston to the crankshaft which in every respect the movement of the connecting rod = piston = piston rod. All movements herein done, are reciprocating and connecting rod is one of the major components of reciprocating masses other than piston, piston rod, pushrods, and other parts of the conventional engine.  


This part is with the piston rod which receives reciprocating movement from the piston. And so, connects, its big end to the crankshaft and small end to the crosshead.   

5. Crankshaft

This is the major, important part of the marine diesel engine. This is where the connecting rod, flywheel, ship's load, and gears; directly or indirectly connected.  Which all types of stresses converge in this part. Looking at the Crankshaft longitudinally, it appears composed of webs where at the front end is the crankshaft gear, and at the other end is the flywheel and load shaft. The most unique purpose of the Connecting rod is it is being a converter. It converts reciprocating motion to a circular one. Due to this conversion, the diesel engine covers a far-reaching niche in terms of being an all-around prime mover in whatever industry that you can. In ships, the diesel engine is used as the main propulsion and to drive both the main and auxiliary generator and visible ancillaries.


As defined, it functions as the converter of reciprocating movement to rotary.  

Below, in Fig 2. is another image that picture-out the parts of a marine diesel engine


Fig 2.

6. Valves

 In small engines it sinks directly through the counterbore in the cylinder head with the valve head rests on the valve seat. which means that the valve head is facing the inside of the cylinder. A valve guide is inserted in cylinder bore and the valve stem passes through the valve guide. The end of the valve stem is secured on the top outside surface of the cylinder and is secured by means of spring retainer, main valve spring, auxiliary valve spring, and valve lock. Near the end of the valve stem is a grooved portion where you can find a circlip which purpose is to prevent the valve from falling inside the cylinder just in case the valve springs break and the valve lock gets loose and out of position. If this happens, the circlip will strike at the tip of protruding valve guide end and will serve as stopper. 
In large engines, the valve is installed in a valve cage and is secured from falling since at the end of the valve stem is threaded and lockscrews secure the valve from falling. The valve cage its cylinder side is where the valve seat is inserted and is movable. therefore, the valve and the cage are one assembly and can be taken out and installed inside its bore of the cylinder by means of valve puller.     

There two kinds of valves employed to control the intake of air to the cylinder and burnt gases expelled out of the cylinder. The control of opening and closing is done by methods that have developed that so many years have passed. A more conventional type is by means of pushrods and then a more modern in the form of hydraulic control and recently by electronic control which is more sophisticated.

There two kinds of valves:
  • Intake valve
  • Exhaust valve


  • Intake valve - controls the admittance of ambient air for combustion. It also seals the cylinder from leaking combustion gases and pressure leaks.
  • Exhaust valve - opens during exhaust stroke to give way of combustion gases out of the engine cylinder. And also keeps the cylinder sealed from leaking during compression stroke.  





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